USB: gadget: gmidi cleanups

Clean up the midi gadget, using newer APIs and conventions:

 - Remove many now-needless #includes

 - Use the DEBUG (from Kconfig+Makefile) and VERBOSE_DEBUG conventions.

 - Whitespace fixes

There should be no effect on object code size.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Ben Williamson <ben.williamson@greyinnovation.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>